This failed in a recent Super User question, which for all posts shows "Post Made Community Wiki by tigerden", who happened to have posted the 16th answer. I've no idea if this is a case where history is flaky, but I doubt it, as the question was only asked 8 hours earlier.
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ArjanJul 24 '13 at 17:24

1

Jarrod, this also failed here, showing "Post Made Community Wiki by user234719" (and same message for all its answers), on June 30 2013, while the question itself dates back to July 22 2009.
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ArjanAug 12 '13 at 12:13

Ah, animuson might have a point here: "the community wiki reason is calculated on the fly, not stored with the post [...] if the question was converted to wiki by having 30 answers and drops back below the 30 mark, the reason disappears and it shows the user who originally posted that 30th answer as having converted the question and its answers to community wiki".
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ArjanNov 21 '13 at 19:50

Today, it failed on all (now: 5) answers on a CW question, where in each answer the author of that very answer is shown in "Post Made Community Wiki by ..." (As usual: just for your information.)
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ArjanDec 26 '13 at 15:55